CHICAGO (AP) _ Confusion descended on downtown today when a series of mechanical problems darkened hundreds of office buildings, snarled traffic and skidded the Chicago Board of Trade to a halt.

Thousands of commuters left early for the day when power was lost, pushing the rush hour forward by hours. Police struggled to keep cars moving through darkened intersections.

Extra buses were called in to handle the crush and generators were set up in Union Station, which handles Amtrak and suburban commuter trains.

Board of Trade officials closed the markets at 1 p.m. after they were warned by utility officials that power was going to be shut down, said board spokesman Bret Gallaway.

``This is a tremendous disruption of trading,'' Gallaway said. ``The last minutes of trading are often some of the busiest trading of the day.''

Paul McCoy, vice president of Commonwealth Edison, said cables leading from two big transformers failed Wednesday night and Thursday morning at a substation just south of downtown, leaving 2,300 customers without electricity.

The utility was forced later Thursday to shut down power to another 670 business customers in a 30-block area after a transformer at another substation overheated. That transformer had been helping to keep up with demand at the defective station south of downtown.

Power to all customers was restored by late Thursday night.

Brian Kelly, a worker at a 13th-floor temp agency across from the federal courthouse, described a chaotic scene when the building's lights went out.

``It was kind of scary. It was so dark and it was chaotic trying to get our stuff before we had to get out,'' he said. ``It was total confusion. It was mass panic.''

``I had to shut down all our computers,'' said Eamon Tierney, a systems manager in a Michigan Avenue building. ``Then I had to walk down 32 flights of stairs.''

At the 27-floor Hilton Chicago & Towers, hotel staff dug into their Y2K emergency stash of fluorescent glow sticks to help evacuate guests who wanted to leave their rooms.

``We didn't think we would have to use them unless the worst Y2K case scenario happened,'' said Robert Allegrini, a spokesman for the hotel. ``Who would have thought they would come in so handy before New Year's Eve?''

Allegrini said they brought out about 6,000 of the glow sticks to light darkened stairwells in the hotel and to hand out to guests trying to find their way down to the lobby.

``It was like being at a rock concert, everybody waving their lights in the air,'' he said.

ComEd is already under fire for an outage during July's brutal heat wave outage that affected about 100,000 customers. In 1995, the utility's customers also suffered outages during that summer's extreme heat, which contributed to the deaths of hundreds of people.

``Like everybody, I'm upset and we expect the answers directly from ComEd,'' Mayor Richard M. Daley said at a news conference. ``We deserve answers.''

Last summer, the utility was a step away from imposing rolling blackouts after a series of problems left it short of power on a sweltering day.

Instead, ComEd asked residential customers to turn off their air conditioners for one- to two-hour intervals, prompting an outcry from customers, a watchdog group and politicians.

``Every summer, it seems to be something different,'' said Rob Kelter, an attorney for the Citizens Utility Board, a frequent critic of ComEd. ``Last summer it was power shortages. This summer it's problems with the transmission and distribution system.''

ComEd officials had been confident similar problems would be avoided this year, pointing in particular to improvements in its nuclear power stations.

John Rowe, CEO of ComEd's parent company, Unicom, called the utility's performance ``absolutely, totally unacceptable'' and said he had appointed a committee to study its problems and recommend how to restructure the company.

``We are going to go at this root and branch,'' Rowe said.

A string of 100-degree days also has caused power problems in Texas.

In Dallas, bankers, jewelers and other merchants were out of business Tuesday, Wednesday and part of Thursday because of power cutbacks at the Dallas Market Center.

Similar problems shut down a shopping mall in Tyler, Texas, and left a Fort Worth hospital without air conditioning.